1. Field of the Invention:
This invention relates to a signal output circuit for an image sensor, and more particularly to an improved signal output circuit for null-adjusting an offset voltage of an operational amplifier incorporated into an integrated circuit together with light receiving elements.
2. Description of the Related Art:
Usually a multiple-tip type image sensor for reading images comprises a plurality of chips on which light receiving elements are juxtaposed. With such an image sensor, the chips tend to show non-uniform electrical characteristics, e.g. offset voltages for an operational amplifier, thereby causing errors in sensor outputs. To compensate for such errors, dark output characteristics of the chips are measured before they are assembled. Then the chips are grouped according to their output characteristics, so that the chips having the same characteristics are selected and die-bonded on a substrate to form an image sensor. The bonded chips are then null-adjusted according to their dark output characteristics by external offset circuits.
A thin film-type image sensor also suffers from non-uniform characteristics in film substrates, so that an assembled image sensor has its offset voltage adjusted by an external circuit.
With foregoing image sensors, the offset voltage of the operational amplifier of the signal circuit should be adjusted by an external circuit. This leads to an increase in the number of components, assembling operations and manufacturing cost.